1. Field of the Invention
The present invention generally relates to a method and apparatus for providing a broadcast service in a mobile communication system for providing wireless packets. More particularly, the present invention relates to a method and apparatus for arranging pilot tones in a broadcast system using an orthogonal frequency division multiplexing (OFDM) transmission scheme.
2. Description of the Related Art
Conventional wireless transmission schemes for broadcast and multicast services (BCMCS) have been developed for the purpose of fixed reception and low-speed mobile reception. Recently, technologies capable of receiving the BCMCS using a small-sized terminal in a high-speed mobile environment have been developed. Broadcast technologies such as digital multimedia broadcast (DMB) and digital video broadcast-handheld (DVB-H) have been developed to receive video-level broadcast using a small-sized portable terminal. Furthermore, research for developing the existing unidirectional broadcast service to a bidirectional broadcast service has been conducted. For the bidirectional broadcast service, a method for exploiting the existing wired/wireless communication network as a return channel has been taken into account. Conventional approach has limitations in implementing basic bidirectional broadcasting because broadcasting and communication are used in different transmission modes.
Service supported in the mobile communication system for providing wireless packets is a communication service for exchanging information between a specific transmitter and a specific receiver. In the broadcast service, different receivers receive information through different channels. However, because isolation between channels is low in the wireless mobile communication system, performance is limited due to interference. To increase the isolation between channels, the conventional mobile communication system uses multiple access schemes such as code division multiple access (CDMA), time division multiple access (TDMA), and frequency division multiple access (FDMA) and a cellular concept. However, because these schemes fundamentally cannot suppress interference, interference is still a factor limiting performance.
On the other hand, the BCMCS service different from the communication service uses a scheme for unilaterally transmitting information from a transmitter to a plurality of receivers. Because users receiving identical information share an identical channel, interference between the users does not occur. In case of a mobile broadcast service, interference due to the multipath-fading phenomenon occurring in a high-speed mobile environment is an important factor capable of lowering reception performance. To overcome this problem, many broadcast systems such as digital video broadcast terrestrial (DVB-T), DVB-H, and digital audio broadcast (DAB) use an OFDM transmission scheme.
An advantage of the OFDM transmission scheme in the broadcast system is that self-interference due to multipath fading can be avoided. Specifically, because different base stations (BSs) transmit identical broadcast signals through a single frequency network (SFN) in the broadcast service, the signals can be received from the different BSs through the OFDM transmission scheme without interference. Accordingly, when the OFDM transmission scheme is applied to the broadcast service, an interference-free environment can be implemented, such that transmission efficiency can be maximized.
A forward link of a high rate packet data (HRPD) system uses a TDMA scheme as a multiple access scheme and uses a time division multiplexing/code division multiplexing (TDM/CDM) scheme as a multiplexing scheme.
FIG. 1 illustrates a structure of one slot in which data is transmitted through the forward link of the conventional HRPD system.
As illustrated in FIG. 1, one slot has the form in which a half-slot structure repeats. NPilot-chip pilot parts 103 and 108 are inserted into the centers of the half slots, respectively. These pilot parts 103 and 108 are used to estimate a channel of the forward link in a receiving terminal. NMAC-chip medium access control (MAC) information parts 102, 104, 107, and 109 including reverse power control (RPC) information, resource allocation information, and so on are transmitted on both sides of the pilot parts 103 and 108. NData-chip data parts 101, 105, 106, and 110 are transmitted on the sides of the MAC information parts. The pilot, MAC, and data parts are multiplexed according to the TDM scheme such that they are transmitted at different times.
On the other hand, data and MAC information are multiplexed according to the CDM scheme using Walsh codes. In the forward link of the HRPD system, a small block size of the pilot part is set to NPilot=96 chips, a small block size of the MAC part is set to NMAC=64 chips, and a small block size of the data part is set to NData=400 chips.
FIG. 2 illustrates a slot structure in which an OFDM block (hereinafter refer to as “OFDM symbol”) is inserted into a data transmission interval of an HRPD forward link slot for the BCMCS.
A position and size of a pilot or MAC signal is set to be the same as those of a pilot or MAC signal in the HRPD slot of FIG. 1 such that HRPD forward compatibility can be maintained. That is, NPilot-chip pilot parts 103 and 108 are placed in the centers of the half slots, respectively. NMAC-chip MAC information parts 102, 104, 107, and 109 are placed on both sides of the pilot parts. Accordingly, the conventional HRPD terminal not supporting an OFDM-based broadcast service can estimate a channel through a pilot, and can receive an MAC signal. OFDM symbols 121, 122, 123, and 124 are inserted into the remaining parts of the slot, that is, data transmission intervals 101, 105, 106, and 110. These OFDM symbols are modulated BCMCS information.
In the conventional HRPD forward link system, NData=400 chips. Also, a size of an OFDM symbol is NData=400 chips. A cyclic prefix (CP) is placed before the OFDM symbol such that self-interference occurring in a received signal delayed through a multipath can be avoided. That is, one OFDM symbol is configured by OFDM data 126, obtained by performing inverse fast Fourier transform (IFFT) on BCMCS information, and a CP 125. A CP size is NCP chips. The CP is obtained by copying an NCP-chip signal from a tail part of the OFDM data and placing the copied signal before the OFDM data. Accordingly, an OFDM data size is (NData−NCP). Here, NCP is determined by an allowable level of a time delay causing self-interference. If NCP is large, a received signal with a large delay is demodulated without interference. However, because an OFDM data size becomes small, an amount of information capable of being transmitted is reduced. On the other hand, if NCP is small, an amount of information capable of being transmitted becomes large but a probability of occurrence of self-interference becomes high, such that reception quality is lowered.
Because identical signals are transmitted from many transmitters in the SFN but a terminal receives the signals at different times, a CP size is conventionally large. In the HRPD forward link system for transmitting an OFDM signal for the BCMCS, it is suitable that NCP=80. In this case, an OFDM data size is 320 chips. This means that 320 modulated symbol elements are transmitted in an OFDM data interval after an IFFT. A total of 320 tones can be ensured through the OFDM scheme.
However, all the 320 tones cannot be used for data symbol transmission. Some tones at the edge of a used frequency band must be used as guard tones for reducing interference to an out-of-band signal. Because the pilot parts 103 and 108 used in the conventional HRPD forward link are spread by codes different between transmitters and transmitted, they are not suitable for the purpose of channel estimation for BCMCS provided in the SFN. Accordingly, a dedicated pilot for channel estimation of an OFDM signal is additionally required. A signal preset between the transmitter and the receiver is transmitted in some tones, such that the transmitted signal is used for channel estimation. These tones are referred to as dedicated pilot tones. Because a relatively large time delay is allowed in the OFDM scheme for the SFN, frequency selective fading may be severe. Sufficient pilot tones must be ensured such that channel estimation can be made also in the severe frequency selective fading. Many types of tones are shown in Table 1. Then total number of tones is 320, the number of guard tones is 16, the number of pilot tones is 64, and the number of tones for transmitting data is 240.
TABLE 1Total Number of TonesNData − NCP = 320Number of Guard TonesNGtone = 16Number of Pilot TonesNPTone = 64Number of Data TonesNDTone = 240FIG. 3 illustrates a conventional tone arrangement method in the HRPD system.
Referring to FIG. 3, guard tones 201 are placed at a band edge. Eight tones corresponding to a half of the 16 guard tones are placed in a low frequency part of the band, and the remaining 8 guard tones are placed in a high frequency part of the band. Because any signal is not transmitted through the guard tones, power is not allocated to the guard tones. Data tones 203 are placed between guard bands. Because pilot tones 202 are used for the purpose of channel estimation, one pilot tone 202 is placed every 5 tones at an equal interval. Four guard tones subsequent to a pilot tone are placed in the lowest frequency part and then the next pilot tone is inserted subsequent to the guard tones.
Also in an area in which the data tones 203 are placed, four data tones 203 are placed subsequent to an inserted pilot tone 202 and then the next pilot tone 202 is placed subsequent to the data tones 203. When the tones are placed according to this method, the pilot tone 205 is placed in a frequency component corresponding to a direct current (DC) component. Because this pilot tone is the DC tone, power is not allocated or low power is allocated to the pilot tone as compared with other tones. Thus, the pilot tones are transmitted at low power.
An amount of power allocated to the pilot tone 202 is different from that allocated to the data tone 203. Because an optimal solution for a power ratio between the pilot tone 202 and the data tone 203 differs according to a channel state, the transmitter and the receiver must define in advance a ratio value.
FIG. 4 is a block diagram illustrating a structure of a conventional transmitter in an HRPD system.
Referring to FIG. 4, the transmitter includes a channel coder 301 for channel-coding received packet data, a channel interleaver 302 for interleaving the coded packet data, a modulator 303 for modulating the interleaved packet data, a guard tone inserter 304 for inserting a guard tone, and a pilot tone inserter 305 for inserting a pilot tone. The transmitter further includes a quadrature phase shift keying (QPSK) spreader 306, an inverse fast Fourier transform (IFFT) processor 307, a cyclic prefix (CP) inserter 309, and a compatible processor 310.
Physical packet data generated from a higher layer is input to the channel coder 301 and is channel coded. A channel-coded bit stream is interleaved through the channel interleaver 302 such that diversity gain can be obtained. The interleaved bit stream is input to the modulator 303 and is converted to a modulated signal. Here, the modulated signal is placed in the data tones 203.
Then, the signal output from the modulator 303 is input to the guard tone inserter 304. The guard tone inserter 304 places the guard tones 201 at boundaries of the band. The pilot tone inserter 305 places the pilot tones 202 at an equal interval. When a signal to be transmitted is allocated to all tones, a QPSK spread process is performed. Through this spread process, different BCMCS contents to be transmitted from BSs are multiplied by different complex pseudo noise (PN) sequences. Here, the complex PN sequence is a complex sequence in which real and imaginary components are configured by PN codes.
Because a signal of an undesired BS affects a receiver in the form of noise, the receiver separates a channel from the undesired BS and performs channel estimation. A complex PN sequence multiplied in the QPSK spread process is generated after a BCMCS content identifier (ID) is input.
After undergoing the QPSK spread process, the modulated signal is placed in a desired frequency tone position through the IFFT process. After undergoing a process for inserting a CP to prevent the effect of self-interference due to multipath fading, an OFDM signal to be transmitted is completed. Then, the pilot parts 103 and 108 and the MAC information parts 102, 104, 107, and 109 are inserted according to a process of the HRPD transmitter. A signal to be finally transmitted has a slot structure as illustrated in FIG. 2.
However, when a pilot tone is placed according to the conventional method, it is placed in the DC component. In this case, there is a problem in that channel estimation around the DC component is difficult. For example, if power is not allocated to the pilot tone 205 of a DC position, it does not match the original purpose in which a pilot tone is placed every 5 tones at an equal interval. Thus, a channel estimation error in data tones 207 and 208 around the DC component is relatively large as compared with the channel estimation error in other positions. A problem of the channel estimation error occurs even when a small power value is allocated to the pilot tone 205 of a DC position.